Coal has been utilized as a fuel for many centuries, usually as it has been taken from the mine. While it has a high heat or B.T.U. value, it has the disadvantage of containing a large amount of contaminating ash. This ash consists of non-combustible materials such as clays, sand shale and other earth derived minerals. It also contains sulfur containing materials, principally pyrite or FeS.sub.2 but also some organic sulfur containing compounds in the coal.
When coal is burned in a modern boiler, as for instance in an electric generating station boiler, several undesirable side reactions accompany the generation of heat. One principal one is the generation of large amounts of ash, some of which exits with the smoke, the rest falling to the bottom of the boiler for removal and disposal. This can be quite a problem since in a modern utility coal is burned at rates up to 2000 tons per hour, and 100 tons per hour consumption rates are quite common. As coal is usually purchased with 10% by weight ash content, the removal and disposal of 100 to 200 tons per hour of ash material, usually of a highly alkaline nature, presents a difficult and expensive problem. There is also a sizable environmental problem to be faced in disposing of the ash due to its high pH.
At the same time, sulfur oxides in the combustion gases are also generated. This is thought to contribute to the problem commonly known as "acid rain" making stack gas scrubbers mandatory on these boilers. The increased disposal load of the sludge from the scrubbers combined with the ash load provide formidable logistical and economic problems to the utility or other user.
When possible, utilities try to purchase coal supplies which are low in ash and sulfur contamination to lessen these problems. In fact, during the decades of the 1950's and 1960's many utilities ceased using coal and turned to oil as a fuel source, mainly to eliminate these problems. It can be appreciated that on the eastern seaboard of the United States with its high population density, the demand for electric energy and scarcity of ash disposal sites combined to make oil an attractive alternate fuel source. With the high price of oil this is no longer such an attractive alternate.
Various prior art process have attempted to solve the heretofore described problems by attempting to remove the ash and sulfur from the coal. For example, one such process using chlorine gas is disclosed in U.S. Pat. No. 4,305,726. Other typical processes are disclosed in the following patents U.S. Pat. Nos.: 4,328,002; 4,224,038; 4,210,422; and 3,993,455. While various prior processes appear to offer some advantages as compared to using a mined coal, they do not appear to have met all of the industries' needs and requirements.